The present invention relates to magneto-plumbite ferrite particles for magnetic cards, a process for producing the same and a magnetic card using the same. More particularly, the present invention relates to magneto-plumbite ferrite particles for magnetic cards, which have an appropriate coercive force and an excellent temperature stability, especially magneto-plumbite ferrite particles in which the change of the coercive force with temperature in the temperature range of -10.degree. to 120.degree. C. is -1.5 to +1.5 Oe/.degree.C., a process for producing such magneto-plumbite ferrite particles, and a magnetic card using such magneto-plumbite ferrite particles.
Magnetic cards. i.e., cards having a magnetized stripe portion wherein data can be stored, have recently spread, as are represented by ID cards, credit cards, railroad tickets, season tickets, highway passes, telephone cards and prepaid railway cards.
Magnetic cards are generally produced by a method of applying a magnetic coating which is obtained by kneading magnetic particles with a vehicle, directly to a card substrate, a method of adhering a magnetic tape which is obtained by applying a magnetic coating to a base film, to a card substrate, or the like.
Such a magnetic card is constantly carried and frequently used both indoors and outdoors throughout the country which have different climates such as temperature and humidity. It is, therefore, necessary that a magnetic card stands use under severe conditions.
In order to produce such a magnetic card, it is necessary to use ferromagnetic particles having an appropriate coercive force and an excellent temperature stability.
As the ferromagnetic particles, magneto-plumbite ferrite particles are generally known.
Since the coercive force of magneto-plumbite ferrite particles is high, it is generally required to reduce the coercive force to 300 to 3000 Oe. In order to reduce the coercive force of magneto-plumbite ferrite particles to an appropriate coercive force, a method of substituting a part of Fe(III) of a ferrite by a substituting element such as cobalt (Co) and titanium (Ti) is proposed.
In addition, magneto-plumbite ferrite particles are strongly required to have a magnetic stability (especially, a stability of the coercive force) with respect to temperature (hereinunder referred to as "temperature stability").
The reason why the temperature stability is important in a magnetic card is as follows. Magnetic recording is performed by magnetizing the magnetic particles in a magnetic card by the magnetic field produced from a magnetic head. The magnetic field of the magnetic head and the coercive force as the magnetizing component of the magnetic particles are adjusted at room temperature so as to enable the optimum recording. However, when the coercive force varies with a rise in the ambient temperature due to a change in the atmosphere, the recording and reproducing characteristics greatly change with the ambient temperature, so that the optimum recording is impossible.
Magneto-plumbite ferrite particles for magnetic cards which have an appropriate coercive force and an excellent temperature stability are thus strongly demanded, but the coercive force of general magneto-plumbite ferrite particles are apt to increase with a rise in the temperature, namely, they are poor in the temperature stability. Since the change of the coercive force with temperature of conventional magneto-plumbite ferrite particles is as large as +4 Oe/.degree.C. they are not appropriate as a magnetic particles for magnetic cards.
Methods of improving the temperature stability of magneto-plumbite ferrite particles in order to solve the above problem are proposed in, for example, Japanese Patent Application Laid-Open (KOKAI) Nos. 62-51026 (1987), 62-132732 (1987), 62-216922 (1987) and 2-296303 (1990).
Japanese Patent Application Laid-Open (KOKAI) No. 62-51026 (1987) discloses a magnetic powder for high-density magnetic recording, comprising hexagonal ferrite particles which have an average particle diameter of 0.02 to 0.2 .mu.m and a coercive force of 200 to 2000 Oe and which contain 0.1 to 1.0 Sn atom based on one chemical formula.
In the specification of Japanese Patent Application Laid-Open (KOKAI) No. 62-51026 (1987), there is a description of "If the average particle diameter is less than 0.02 .mu.m, the magnetization and the coercive force reduce so much as to reduce the recording output of a magnetic recording medium, while if the average particle diameter exceeds 0.2 .mu.m, the coercive force reduces and the noise at the reproduction greatly increases, and they are reasons that the average particle diameter of the uniaxial anisotropic hexagonal barium ferrite crystals is restricted to 0.02 to 0.2 .mu.m."
Japanese Patent Application Laid-Open (KOKAI) No. 62-132732 (1987) discloses a method of reducing the temperature dependency of the coercive force of magneto-plumbite ferrite particles by restricting the average particle diameter to not more than 1.0 .mu.m, the thickness of a particle in the direction of C-axis to not more than 0.2 .mu.m, and the aspect ratio of a particle represented by the following formula to not less than 5: ##EQU1##
In the specification of Japanese Patent Application Laid-Open (KOKAI) No. 62-132732 (1987), there is a description of "The average particle diameter of magneto-plumbite ferrite particles used for a product utilizing magnetism is generally not more than 1.0 .mu.m and the particles having a larger diameter do not provide predetermined magnetic characteristics, and this is a reason that the average particle diameter of magneto-plumbite ferrite particles of the present invention is restricted to not more than 1.0 .mu.m."
Japanese Patent Application Laid-Open (KOKAI) No. 2-296303 (1990) discloses a magnetic powder for magnetic recording, which is represented by the following general composition formula: EQU (Fe).sub.a (Sn).sub.b (Si).sub.c (M.sup.1).sub.d (M.sup.2).sub.e (M.sup.3).sub.f (O).sub.g
(wherein M.sup.1 represents at least one metal element selected from the group consisting of Ba, Sr, Ca and Pb, M.sup.2 represents at least one metal element selected from the group consisting of Cr, Y, Ce, Nd, Sm, Al, La and Cd, M.sup.3 represents at least one metal element selected from the group consisting of Mg, Ti, Mn, Ni, Cu, Zn, Sb, In, Mo and W, and a, b, c, d, e, f and g respectively represent the number of atoms of Fe, Sn, Si, M.sup.1, M.sup.2, M.sup.3 and O, wherein a is 8.0 to 12.0, b is 0.01 to 6.0, c is 0.05 to 6.0, d is 0.3 to 6.0, e is 0.01 to 6.0, f is 0.0 to 6.0, and g is the number of O atoms which satisfies the atomic values of the other elements), and which has an average particle diameter of not more than 0.08 .mu.m and a plate ratio of not more than 8.
In the specification of Japanese Patent Application Laid-Open (KOKAI) No. 2-296303 (1980), there is a description of "Since the average particle diameter is as small as not more than 0.08 .mu.m, the magnetic powder is suitable for high-density recording, and since the plate ratio is as small as not more than 8, especially, 2 to 5, the packing density in the coating medium is high and it is possible to obtain a magnetic powder having a large saturation magnetization and an excellent dispersibility."
Japanese Patent Application Laid-Open (KOKAI) No. 62-216922 (1987) discloses fine hexagonal ferrite particles for magnetic recording which is represented by the following general formula: EQU AFe.sub.(12-x-y-z) Co.sub.x Ti.sub.y M.sub.z O.sub.19
(wherein A represents at least one element selected from the group consisting of Ba, Sr and Pb, M represents at least one substituting element selected from the group consisting of Sb, Sn, W and In, x is 0.5 to 1.1, y is 0.1 to 0.5, and z is 0.1 to 0.5), and which has an average particle diameter of 0.01 to 0.2 .mu.m.
In the specification of Japanese Patent Application Laid-Open (KOKAI) No. 62-216922 (1987), there is a description of "If the average particle diameter of fine hexagonal Co, Ti, M-substituting ferrite particles is less than 0.01 .mu.m, the particles have superparamagnetism so that they are unsuitable as a magnetic recording material, while if the average particle diameter exceeds 0.2 .mu.m, the modulation noise increases so that they cannot be used as a magnetic recording material, and therefore, the fine particles having an average particle diameter of 0.01 to 0.2 .mu.m are selected. From the view point of the improvement of the packing density and the S/N, the average particle diameter is preferably 0.01 to 0.1 .mu.m."
The method disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 62-51026 (1987) improves the temperature stability by adding a predetermined amount of Sn to a hexagonal ferrite. However, the average particle diameter of the hexagonal ferrite particles obtained is so small as 0.02 to 0.2 .mu.m that they are not desirable as magnetic particles for magnetic cards when the recording density, the output, etc., are taken into consideration. As to the change of the coercive force with temperature, when the temperature stability of the hexagonal ferrite particles disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 62-51026 (1987) is measured by a method adopted in the present invention, it is as large as -2 to +4 Oe/.degree.C. That is, the improvement of the temperature stability by the method cannot be said satisfactory.
In the particles disclosed in Japanese Patent Application Laid-Open (KOKAI) Nos. 62-132732 (1987), 62-216922 (1987) and 2-296303 (1990), the change of the coercive force with temperature is also large when measured by a method adopted in the present invention, so that the particles have proved to have a poor temperature stability.
Accordingly, the technical problems of the present invention are to provide magneto-plumbite ferrite particles for magnetic cards which have an appropriate coercive force and a more excellent temperature stability.
As a result of various studies undertaken by the present inventors so as to solve the above-described problems, it has been found that to produce magneto-plumbite ferrite particles which are represented by the following general formula: EQU AO.multidot.n{(Fe.sub.1-(a+b) Bi.sub.a M.sub.b).sub.2 O.sub.3 }
(wherein A represents at least one metal selected from the group consisting of Ba, Sr and Ca, M represents either Co and Sn or Co, Ti and Sn, n is 5.5 to 6.1, a is 0.001 to 0.010 and b is 0.010 to 0.200), by mixing a compound containing at least one metal selected from the group consisting of barium, strontium and calcium, an iron oxide, a Bi compound, and either a Co compound and an Sn compound or a Co compound, Ti compound and an Sn compound in the above-described stoichiometric amount, and heat-treating the resultant mixture in the presence of a flux of 0.1 to 20 wt % based on the iron oxide at a temperature of not less than a melting point of the flux, the thus-obtained magneto-plumbite ferrite particles have -1.5 to +1.5 Oe/.degree.C. of the change of the coercive force with temperature in the temperature range of -10.degree. to 120.degree. C., an appropriate coercive force and an excellent temperature stability. On the basis of this finding, the present invention has been achieved.